It is known that an underwater vessel (i.e., a submarine) generates sound as it travels through the water. The sound is generated by a variety of sources, including, but not limited to, sound generated by a submarine propulsion system, sound generated by a submarine propeller, and sound generated by a submarine electrical power generator. It is known that submarine designers attempt to reduce these and other sound sources in order to make a submarine difficult to detect by passive acoustic means, therefore remaining as covert as possible.
Some water-born objects do not emit sound, for example, underwater mines. These objects cannot be detected by the sounds they make.
While a conventional passive sonar system merely listens for sounds made by a target of interest, a conventional active sonar system transmits a burst of acoustic energy, called a “ping,” which travels at the speed of sound through the water. Some of the acoustic energy reflects from objects in or on the water back toward the active sonar system. These reflections, referred to as “echoes,” are received by acoustic sensors at the active sonar system. After a sufficient amount of time has passed, the active sonar system transmits another ping and repeats the above process.
Both active and passive sonar systems must operate in an environment filled with acoustic noises generated by a variety of noise sources, including, but not limited to, ships, surface waves, wind, geologic noises, and biologic noises.
Detection electronics, which forms a part of the active sonar system, performs processing upon the received echoes to improve the likelihood that only echoes from targets of interest are identified and reported to a sonar system operator. However, as described above, the undersea acoustic environment is very noisy, and despite the application of sophisticated detection processing algorithms, the active sonar system may still falsely identify random bursts of noise as targets. These false detections are referred to as “false alarms.” If the consequences of reporting a false alarm are severe, then steps can be taken to further reduce a probability of the false alarms, but usually these steps also reduce the probability that a real target of interest will be detected.
A variety of approaches have been used in sonar systems to improve performance in the presence of the noisy ocean environment. For example, both active and passive sonar systems tend to do receive and/or transmit beamforming. Receive beamforming, for both passive and active sonar systems, tends to result in blocking out of directions from which noises may come. Transmit beamforming, for active sonar systems, tends to result in higher power in a transmit beam, and therefore, a stronger echo from an object in or on the water.
Another approach used in sonar systems to improve performance is a matched-filter technique, which will be understood to those of ordinary skill in the art to take a variety of forms in the time or frequency domains.
Another approach used in active sonar systems to improve performance is a “chaining algorithm” that attempts to identify echoes that appear in adjacent ping cycles at ranges consistent with a real target moving at a realistic speed.
It is known that sound can travel through the water in so-called “propagation paths,” which can be non-straight paths, particularly when the propagation paths extend over appreciable distances, e.g., miles. The propagation paths can be modeled with propagation models. Some propagation models assume that the sound travels in straight propagation paths. These models are often referred to as isovelocity models, since they presume that sound travels at the same sound speed at all water depths. Other propagation models do not assume that the sound travels in straight propagation paths. These models, which are sometimes referred to as “ray trace” models, can be used to more accurately predict the sound propagation paths and the resulting sound that arrives at a point in the ocean, for example, at a sonar system that receives passive sound from an underwater target. Other propagation models accomplish the equivalent function but are less computationally convenient.
As is also known, sound that travels underwater can often take more than one propagation path. For example, sound can take a “direct propagation path” from a sound source to a sound receiver, which path may curve but not intercept the surface or bottom of the ocean. The sounds can also travel upward from the sound source, on a so-called “surface reflected path,” reflecting (or scattering) from the surface of the water and traveling downward to the sound receiver. The sound can also travel downward from the sound source, on a so-called “bottom reflected path,” reflecting (or scattering) from the bottom of the water basin and traveling upward to the sound receiver. The sound can also take a variety of other propagation paths, having, for example, both a surface and a bottom reflection (or scattering) or more than one surface and bottom reflection (or scattering).
Through there exist a very large number of sound propagation paths between a sound source and a sound receiver, some of the propagation paths are dominant, i.e., sound received at a sound receiver will have an intensity largely from the dominant sound paths. In particular, because sound tends to lose intensity each time it reflects or scatters from the surface or the bottom, the propagation paths having the strongest sound intensity when received at a sound receiver tend to be the direct path, the surface reflected path, and the bottom reflected path. However, a surface to bottom reflected path and a bottom to surface reflected path can also be considered as well as paths with multiple boundary contacts.
Conventional active sonar systems tend to operate with direct sound paths between the active sonar system and the target of interest. However, conventional active sonar systems also experience (i.e., receive) sound reflecting from the ocean surface and from the ocean bottom. Active sonar systems must distinguish an echo from a target from a reflection from the oceans surface or from the ocean bottom.
It would be desirable to provide new approaches used in active sonar systems to improve performance in the presence of the noisy ocean environment. For example, it would be desirable to provide improved detection and localization of objects in the water.